Mexico closed its schools across its capital Friday after at least 16 otherwise healthy people died and more than 900 others fell ill from what could be a new strain of swine flu. The World Health Organization worried that it could mark the start of a flu pandemic. Scientists in the U.S. and Mexico were trying to determine if the deaths were due to the same new strain of swine flu that sickened seven people in Texas and California.

The World Health Organization counted at least 57 deaths in Mexico, although it wasn’t yet clear if this larger number was due to swine flu. “We are very, very concerned,” WHO spokesman Thomas Abraham said. “We have what appears to be a novel virus and it has spread from human to human.” If international spread is confirmed, that meets WHO’s criteria for raising the pandemic alert level, he added.

WHO also raised its internal alert system Friday, enabling the agency to divert more money and personnel to dealing with the outbreak. “It’s all hands on deck at the moment.” Abraham said. Mexico’s Health Secretary, Jose Cordova, said only 16 of the deaths have been confirmed to have been caused by the new strain, through testing at government laboratories. Samples from 44 other people who died were still being tested. The health department put the total number of people sickened at around 943 nationwide.